Silylene heterocyclic

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

This silylene formation from 27 under mild conditions permits the synthesis of a variety of interesting carbo- and heterocycles, most of which are new types of compounds. The results are summarized in Schemes 5 and 6. The reactions with benzene and naphthalene represent the first examples of [2+1] cycloadditions of a silylene with aromatic C=C double bonds.59 623 The reactions with carbon disulfide and isocyanide (Scheme 6) are also of great interest because of their unusual reaction patterns.62b... 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

A new type of six-membered A-heterocyclic stable silylene has been reported recently. The yellow crystalline [SiCL )] (L = N(R)C(Me)CHC(=CH2)NR, R = C6H3P 2-... 

The nitrogen heteroatoms in imidazole and some closely related heterocycles can stabilize a carbene center at the 2-position (97AG(E)2162). Thus, 1,3-disubstituted imidazole-2-ylidenes (163)-(170), l,3-dimesitylimidazoline-2-ylidene (171), 1,3,4-triphenyl-1H-1,2,4-triazole-5-ylidene (172), and their silylene (173) and germylene (174) analogues are stable (in the absence of oxygen and moisture) solids with definite melting points, which can be recrystallized from appropriate hydrocarbon solvents. The exception is carbene (163) which is an unstable liquid however, it is stable in solution. 

The silylene Tbt(Ph)Si reacts with selenium to give the 6-membered heterocycle 30 through insertion of silylene, once the 5-membered derivative 31 has been formed... 

Table 2 Experimental and calculated geometries of N-heterocyclic silylenes using different methods (bond lengths in pm, bond angles in degrees)...

The molecular structures of. V-heterocyclic silylenes 77-80 having different substituents at the ring nitrogen atoms were computed at several levels of sophistication (see Table 2). The theoretical results obtained using different methods are not only very similar but also close to the experimental results. 

Table 3 Experimental ionization potentials of A/-heterocyclic silylenes...

The electronic structures of the l,3,2(A2)-diazasiloles 83-85 and 89 have been probed by He(i) and He(n) photoelectron spectroscopy. The experimental ionization potentials are summarized in Table 3. The molecular orbitals were assigned with the help of quantum mechanical computations for model compounds and in each case the HOMO was found to be a Jt-type MO of b, symmetry. In the case of the benzol] and pyiido[/4 fused compounds 85 and 89 also the HOMO-1 is a Jt-type MO. The next band was assigned to a lone pair-type MO of aj symmetry, which is mostly located at the dicoordinated silicon atom. Therefore, the HOMOs of the cyclic silylenes 83 and 84 differ in nature from those of the homologues V-heterocyclic carbenes which consist of an essentially aj lone pair-type MO at... 

The analysis of core excitation spectroscopy for the V-heterocyclic silylenes 83 and 84 indicates significant 7t-allyl delocalization over the N-Si(A2)-N fragment with additional cyclic conjugation in 83 <19980M2352, 19990M1862>. 

The reaction of silylenes 83 and 85 with the chalcogens S, Se, Te resulted in the formation of the respective four-membered heterocycle 102 (Scheme 8) <1996JOM211, 1998JA12714>. For the reaction of silylene 83 with 1 equiv of sulfur low-temperature NMR studies suggest the formation of silanethione 103 which then dimerizes. Reaction of excess sulfur with silylene 83 results in the formation of compound 104 with simultaneous release of the diimine ligand. 

Irradiation of the latter product yields a six-membered ring with one Si-Si linkage, and a heterocyclic silylene... 

Note that in the investigations of SiF2 (see Silylenes) a heterocyclic compound was found with carbon as the heteroatoms. 

The nitrogen analog of oxasilacyclopropane, azasilacyclopropane (or silaziridine), has been postulated as a potential reactive intermediate in photochemical- or thermal processes involving silylenes. These strained A-heterocycles can be accessed from the... 

Summary Copyrolytic gas-phase reactions of l,l,2,2-tetra(alkoxy)- and tetrakis-(dimethylamino)dimethyldisilanes with dienes, heterodienes, and heterotrienes in a flow-system furnish a number of unsaturated silicon heterocycles via silylene-intermediates with the exception of 1,4-diaza- and 1,4-oxazadienes, yielding 40-65 % of 1,3,2-diaza- and l,3,2-oxazasilacyclopent-4-enes, respectively, mixtures of diastereoisomeric 1-oxa- or 1-aza-2-silacyclopent-4-enes and -3-enes are formed. Conjugated oxatrienes were found to undergo additionally [6+l]-cycloadditions giving l-oxa-2-silacyclohepta-4,6-dienes. 

A zwitterionic (see Zwitterion) carbene-stannylene adduct has been prepared via the cleavage of a dibenzotetraazafulvalene by a stannylene. Thus, reaction of the tetraamine l,2-C6H4-[NHCH2CH2NMe2]2 with bis(bis(trimethylsilyl)amido)tin(II) yielded the N-heterocyclic stannylene (85). Treatment of stannylene (85) with A.MM A -tetramethyl dibenzotetraazafulvalene results in C=C bond cleavage in the dibenzotetraazafulvalene to give the carbene-stannylene adduct (86). An X-ray structure determination showed that compound (86) is zwitterionic with a partially cationic carbene subunit and a partially anionic stannylene unit. The structure of (86) is similar to that of the carbene-silylene (80) such that the carbene plane and the stannylene plane are orientated in an almost perpendicular manner. 

Si-Heterocycles possessing chiral silicon centers, synthesis of 84MI18. Silylenes as precursors of Si-heterocycles 90UK918. 

Summary Anellation effects on A -heterocyclic silylenes, analogous carbenes, and germylenes are compared. Based on the trends observed, properties of new target molecules are estimated. 

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